There are many different technologies suitable for detecting intruders entering a site. Two technologies that are regularly used in the current security industry are passive infra-red (PIR) detection, and video motion detection (VMD).
Detectors including PIR sensors measure the intensity of heat at wavelengths that match those of the heat emitted by warm blooded animals including humans. If this heat intensity differs from the background heat intensity, then the detector can detect the change as the animal passes in front of the sensor, and raise an alarm. If two PIR sensors are used in tandem, a differential signal may be produced and detected as the target crosses first one and then the other sensor. This increases the signal reliability and compensates for ambient temperature changes.
PIR detectors target all objects that have a different temperature to the background temperature and cross into a detection area (field of view) or a virtual curtain. Because a hot target at a distance can give a similar signature to a cooler target nearby, the maximum range of a PIR is unbounded, and sometimes needs to be “terminated” using a physical barrier just beyond the maximum desired detection distance. Conversely, PIR detectors are insensitive to targets that have a similar temperature to the background, such as a person wearing a wet coat on a wet day. Furthermore, since the detection pattern is fixed by the selected optics, it is not configurable and may not be optimal for a particular intrusion event.
VMD is performed by computer software on a sequence of digital images captured by a video camera that is monitoring the scene of interest. Each image in the sequence is composed of an array of picture elements (pixels). Targets such as intruders typically show up as a different intensity to the background scene in the image and VMD detects intruders in the sequence by looking for changes in pixel intensities that are consistent with a target moving through the scene. Groups of pixels associated with a target are tracked from frame to frame to determine the direction of motion. If the scene is calibrated, the size of the target, the distance it has travelled, and the speed of its travel can be estimated from the tracked group of pixels. By ignoring targets that do not meet size, speed, distance travelled and direction criteria, the security system can be tuned to detect human and vehicle movement while rejecting small animal and foliage movement. However, while many targets can be filtered out by these methods and ignored, some cannot. VMD is sensitive to all changes of intensity in the scene so it can potentially detect moving shadows, moving headlights and/or the beams of light they project onto the scene, moving foliage, animals, birds, humans, and vehicles. Those changes that are not caused by humans or vehicles can create false alarms that consume time and money for monitoring stations responsible for the security of the premises. Conversely, if the target cannot be clearly seen due to poor contrast between target and background, due to poor lighting or inclement weather for example, then VMD may fail to detect it. Neither outcome is desirable for a security system.
In order to improve detection performance and reliability combined PIR and VMD detection systems have been employed. To reduce the occurrence of false alarms from either technology, it is common practise to perform a logical AND operation on the VMD and PIR outputs to produce an alarm only if both detection methods go into alarm. This is called a “double-knock” system.
If VMD and PIR detectors are used together in a double-knock arrangement, the combined system has better false alarm rejection (it ignores false alarms that affect only one type of detector), but a reduced detection capability (it may not detect targets that a system with one type of detector could detect). It is also still possible to produce false alarms if both detectors detect different false alarms simultaneously. A further problem is that the fields of view of the PIR detector and the VMD may not be identical which also reduces the validity of the double knock configuration. An alternative configuration is to accept all alarms from both detectors (logical OR operation). This increases the detection capability as all detections cause alarms, but also increases the false alarm rate as false detections from either detector cause alarms.
The problem across all these permutations is poor detection system performance either through too many false alarms or too many missed detections. The present invention aims to provide a detection system with improved detection performance and fewer false alarms.
Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.